Installing XThreads - cannot modify header information?

[MsMaples]

Thank you in advance if you can help me solve this one!

I am admittedly a newbie - I just installed MyBB today. I am having trouble installing XThreads. Once I extracted the files, I dragged folders under "Upload" into my root folder (public_html), and they seem to have sorted into the appropriate folders.

Based on my searches here, I have checked "class_page.php" for extra spaces on line 151 and did a find replace in "xthreads_attach.php" for require and replaced with require_once, neither did anything.

When I go into Configuration > Plugins I get the following wall of text:

Warning [2] Cannot modify header information - headers already sent by (output started at /home/username/public_html/admin/inc/class_page.php:151) - Line: 24 - File: inc/plugins/xthreads_attach.php PHP 5.6.31 (Linux)

File Line Function
[PHP] errorHandler->error
/inc/plugins/xthreads_attach.php 24 header
/inc/plugins/xthreads_attach.php 96 fatal_error
/inc/plugins/xthreads_attach.php 382 do_processing
/admin/modules/config/plugins.php 518 require_once
/admin/index.php 821 require

Warning [2] Cannot modify header information - headers already sent by (output started at /home/username/public_html/admin/inc/class_page.php:151) - Line: 25 - File: inc/plugins/xthreads_attach.php PHP 5.6.31 (Linux)

File Line Function
[PHP] errorHandler->error
/inc/plugins/xthreads_attach.php 25 header
/inc/plugins/xthreads_attach.php 96 fatal_error
/inc/plugins/xthreads_attach.php 382 do_processing
/admin/modules/config/plugins.php 518 require_once
/admin/index.php 821 require
Can't find XThreads base directory.